Typically, most of the materials expand with heat and contract with cold with the change of the outside temperature, but some materials will contract with a rise of temperature, i.e. having negative thermal expansion (referred to as NTE) behavior. However, the NTE materials found in early stage, such as perovskite ferroelectrics PbTiO3, semiconductor material Si, ice, quartz, cordierite, 2MgO.2Al2O3.5SiO2 and zeolite, have disadvantages of narrow range of negative thermal expansion and anisotropism and there are many difficulties in practical application, so these materials have not attracted widespread attention. Until in 1995, the Sleight research team, University of Oregon, found that ZrW2O8 has isotropic NTE effects in the temperature range of 0.3˜1050K and the thermal expansion coefficient is up to −8.7×10−6K−1, and the research institutes all over the world successively carried out the research on NTE materials. Currently the biggest use of NTE materials is combination with positive thermal expansion materials to prepare composite materials with adjustable thermal expansion coefficient, and the NTE materials can be widely used in the electronics, optics, microelectronics, optical communications and daily life fields. But there are two major problems badly in need of solution when using negative thermal expansion materials and positive thermal expansion materials to prepare composite materials: phase transition and random morphology problems of ZrW2O8. ZrW2O8 will occur α-β temperature phase transition at about 430K, the thermal expansion coefficient of which greatly changes from −8.8×10−6K−1 to −4.7×10−6K−1 and which will lose effectiveness because ZrW2O8 will produce great thermal mismatch stress with the matrix in the process of actual application; ZrW2O8 obtained by the traditional solid phase methods has no regular morphology and also has sharp edges and corners, which easily causes local stress concentration, resulting in the generation of microcracks and affecting its practical application. Therefore, it has great significance to prepare isotropic negative thermal expansion materials with regular shape and no phase change in the range of serviceability temperature.
Studies have shown that: W is replaced by Mo to form ZrW2-xMoxO8 series NTE solid solution compounds, and the phase transition temperature is decreased with the change of W/Mo ratio, in particular with the increase of the Mo substitution amount, for example, the phase transition temperature of ZrWMoO8 is 270K and the scope of application is expanded below room temperature. Currently, through the systematic research on the structure and nature of the ZrW2-xMoxO8 system, looking for a new NTE material is still a main task in the field of negative thermal expansion materials. According to the theoretical speculation, the phase transition temperature of ZrW0.5Mo1.5O8 will be lower, and the application scope also becomes large.
At the present time, there are only a very few reports about controlling the morphology of the NTE materials. Chinese patent ZL200610040201.X used the reflux method to control the morphology of ZrWMoO8 powder, but the reflux method needed long synthesis time and rather complicated procedures and the process was difficult to control; Cora Lind, University of Toledo, in the article “Leah C. Kozy, Muhammad Nawaz Tahir, Cora Lind, Wolfgang Tremel. Particle size and morphology control of negative thermal expansion material cubic zirconium tungstate[J]. J. Mater. Chem., 2009, 19, 2760-2765” reported that using hydro-thermal method, the reaction conditions were strictly controlled to achieve the morphology control of ZrW2O8, and compared with the reflux method, the hydro-thermal method greatly reduced the experimental period (several hours) because its special reaction conditions were conductive to the formation and growth of the crystal. Cheng Xiaonong, et al., from Jiangsu University in the article “Yang juan, Cheng Xiaonong, Liu Qinqin. A method for preparing negative thermal expansion material ZrW1.7Mo0.3O8 single crystal [p]. ZL200510011252.5” reported using hydro-thermal method to prepare ZrW1.7Mo0.3O8 single crystal, and in the article “Q Q Liu, J Yang, X J Sun, X N Cheng. Morphology control and negative thermal expansion in cubic ZrWMoO8 powders[J]. Physical Status Solidi B-basic Solid State Physics, 2008, 245(11), 2477-2482” and “Q Q Liu, J Yang, X J Sun, X N Cheng. Preparation and characterization of ZrWMoO8 powders with different morphologies using hydrothermal method [J]. Journal of Materials Science, 2007, 42(7), 2528-2531” reported the research on using hydro-thermal method to control the morphology of ZrWMoO8, and because of the specificity of ZrW2-xMoxO8, it is found experimentally that the preparation methods of the materials with different W/Mo ratio, especially the process for controlling morphology, are not repetitive, and at present, there is no research report on the morphology control of ZrW0.5Mo1.5O8.